This invention relates to pistons for internal combustion engines and to the manufacture thereof. The invention is particularly concerned with the manufacture of a strong piston that is also light in weight and suitable for cost-effective mass production for small capacity, high compression engines.
Pistons for internal combustion engines for mass market automobiles are manufactured in large numbers and subjected to cost constraints, which in turn place limits on manufacturing processes. Such pistons are usually, but not necessarily, cast from a light metal alloy, typically aluminium based, and then subjected to a series of machining steps that culminate in a precision component.
For heavy duty use, for example in compression ignition engines, it is known to manufacture pistons of steel, usually forged, but such pistons have tended to have a weight penalty, notwithstanding extensive machining operations to remove extraneous metal, and have thus far been restricted to large capacity, low-revving engines found in trucks and the like.
In recent times there has been a need to provide such compression ignition engines for use in smaller, automobile engines, where it is necessary to run at higher speeds and with such high compression pressures. Although steel is a material having suitable properties, and has such strength that it could be used in relatively thin sections that mitigate most if not all of the weight penalty, there is difficulty in manufacturing a small one-piece piston that is capable of fulfilling such potential. In general such a piston has to be manufactured in steel by forging, with attendant limits to wall thicknesses and shapes that limit weight reduction.
It has been proposed to assemble or construct a steel piston from separately manufactured parts, as in U.S. Pat. No. 1,667,202 and U.S. Pat. No. 2,244,008. However the crown forms of the pistons shown therein are relatively simple in structure and without an in-crown combustion bowl often required by modern engines. Even without such added complexity, it is believed that the number of separate parts and assembly operations required are not conducive to providing a small piston capable of operating within a modern small engine in a cost-effective manner.
Notwithstanding that a small piston for mass production is subjected principally to constraints of cost, a larger piston for heavy duty application is subjected principally to constraints resulting from weight, so that the ability to produce a light weight piston cost effectively is not restricted in applicability. With this in mind, it is an object of the present invention to provide an engine piston of assembled form that is capable of providing strength and light weight in simple form and a method of producing such a piston that is capable of implementation more cost-effectively than hitherto.
According to a first aspect of the present invention an engine piston comprises an outer shell, including a crown centred on a longitudinal piston axis and a tubular side wall extending axially with respect to the periphery of the crown to an open end and, within the tubular side wall of the shell, a mounting member arranged to extend transversely to the longitudinal axis and bonded both to the crown and to the side wall spaced from the crown, and gudgeon pin boss means carried by the mounting member.
The term “longitudinal axis” is employed in relation to defining the piston with respect to the geometric centre of the crown, and notwithstanding that the cross section of the piston is other than circular, for example, is to a small extent elliptical or oval.
Preferably, the tubular side wall includes, adjacent the crown, a region of axially spaced, circumferentially extending ring grooves and the mounting member is bonded to the side wall at the end of the ring groove region remote from the crown and the periphery of the mounting member is bonded to the peripheral side wall substantially at the same axial position as at least one ring groove.
More preferably, the peripheral region the crown, the tubular side wall and the bonded mounting member define therebetween an annular cooling chamber.
Preferably, the gudgeon pin boss means is provided integrally with the mounting member, but notwithstanding this the bonding of the mounting member to the crown and to the side wall displaced from the crown create a monocoque type of structure which includes the ring groove region and provides great pressure resistance therefor without need for substantial wall thickness.
According to a second aspect of the present invention a method of manufacturing an engine piston comprises forming an outer shell part comprising a crown, centred on a longitudinal axis, and a tubular side wall, extending axially with respect to the periphery of the crown to an open end, forming a mounting member, carrying gudgeon pin boss means thereon, with a periphery dimensioned to fit within and interface with the tubular side wall, disposing the mounting member within the tubular side wall such that it interfaces with the crown at a crown interface and interfaces with the side wall at a wall interface and bonding the mounting member to the shell at said crown and wall interfaces.
Preferably, the method comprises forming the tubular side wall and at least the peripheral part of the crown, bounding a central crown region, as a integral shell body.
Preferably the outer shell and mounting member are provided separately, as unitary or pre-assembled bodies and are then bonded metallurgically to form the piston.
In this specification references to bonding metallurgically are intended to mean all known techniques employed in joining metal bodies directly to each other or by way of an intervening metal, and includes brazing and various forms of welding, such as friction welding and laser or other beam or jet welding.
The outer shell body may be formed by back extrusion or forging. Alternatively, the outer shell body may be formed by flow forming.
If, as may be considered the norm, combustion bowl means is required in the crown this may be formed integrally with the shell body, subject to shape constraints, or may be formed separately and metallurgically bonded.
The outer shell part and/or mounting member may be made from steel which is suitably ductile, but the method is equally applicable to ductile alloys of non-ferrous materials.
These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment. The drawings that accompany the detailed description are described below.